1. Field of the Invention
The present invention relates to the field of flash memories.
2. Prior Art
Flash memory has the advantages of being selectively erasable, relatively inexpensive nonvolatile memory, and is used in a very wide variety of products to retain information after power is shut off. Currently the mainstream flash technology is referred to as ETOX (erasable tunnel oxide) which uses Channel hot electron (CHE) programming (FIG. 1) and Fowler Nordheim (FN) tunneling for erasing (FIG. 2). The programming mechanism (CHE) is fast, but consumes a lot of power (˜300 to 500 μA per bit), while the erasing mechanism (FN) has a low power dissipation (˜1 pA/bit). In portable systems, there is a need for non-volatile memory with low power consumption for both programming and erasing.
FIG. 3 illustrates a typical ETOX cell layout. In this Figure, regions 20 and 22 are contact regions between a voltage sources and an active region below. Specifically, region 20 provides a contact between the Vss line and the common source lines. In that regard, all cells share a common source. Each cell shares its drain contact with the adjacent cell on the same bit line through contacts 22. Access transistors for the bit lines BL1 and BL2 are not shown. The word lines WL1 and WL2 extend across the entire array and form the gate lines, with floating gates 24 between the word lines and the active regions there below. Table 1 shows the voltages required to selectively program cell A.
TABLE 1BL1BL2WL1WL2SourceProgram A6 V12 VVs.Program Inhibit BVs.12 VVs.Program Inhibit C6 VVs.Vs.Program Inhibit DVs.Vs.Vs.Block Erase all cellsfloatfloatVs.Vs.12 V
Also known are NAND flash memories. These memories are programmed and erased using Fowler Nordheim tunneling. In these memories, typically 8 or 16 cells are connected in series, drain to source, drain to source, with each NAND block being programmed serially. The NAND cells have the advantage of small size, but characteristically have long random access read times.